Analog-to-digital converters of this kind are known for example through the patents DE 28 20 601, and corresponding U.S. Pat. No. 4,361,831, DE 39 21 976 and corresponding U.S. Pat. No. 5,066,955. Due to their continuous integration of the input signal in time, i.e. integration without any need to switch the input signal, these procedures are especially suitable for highly accurate and stable analog-to-digital conversion. Furthermore, a class of one bit analog-to-digital converters is known, the so-called delta sigma converters, which aroused interest specially for digital voice signal processing in conjunction with frequency-dependent shaping of the quantization noise followed by digital filtering. The disadvantages of the latter procedure are the poor long-term and short-term stability and temperature drift caused mainly by the frequent switching, and the relatively large background noise level.
Both groups have in common the possibility of distributing the quantization noise power over a large frequency range by oversampling the input signal with a sampling frequency greater than the Nyquist frequency resp. Nyquist rate (cf. Ref. 8, p.209). This gives the opportunity of removing a large fraction of this noise power subsequently by digital low pass filtering (Ref. 5, Ref. 2 and Ref. 7).